Advances in metal working equipment manufacture, and economic necessities of higher producitivty have combined to put greater demands of improved performance on cutting tool materials. At higher cutting speeds, the chemical inertness of a cemented carbide tool as well as its strength are compromised due to higher temperatures generated from high speed machining. Even with coatings of refractory compounds such as aluminum oxide, the cemented carbide cutting tools have reached the useful limit due to thermal deformation of the tool tip, resulting in poor machining performance and tool life.
Conventional ceramic cutting tools such as aluminum oxide, aluminum oxide/titanium carbide composites and the like overcome many of these problems because of their superior thermal deformation resistance. They, however, suffer from low impact strength and fracture toughness. Silicon nitride based cutting tool materials are found to have superior fracture toughness compared to alumina based cutting tools, but show lower chemical wear resistance when cutting steel.
In an effort to obtain the improved fracture toughness and chemical wear resistance desirable, silicon nitride based cutting tools have been coated with carbides, nitrides and carbonitrides of titanium, hafnium, and zirconium, as well as with aluminum oxide.